Drops, Bubbles and Wetting in Helium

****非技术摘要****靠近固体表面的原子受到远距离的引力。分子间的长程力很重要，因为它们决定了物质的宏观性质；比如哪个相（固体、液体、气体等）在给定温度下是稳定的，以及滑动表面之间的摩擦。远程力还决定了蛋白质和DNA等重要生物分子的形状。这个个人研究者奖支持的项目旨在测量远程力在一些简单的模型系统使用几种互补的方法。一种方法是将固体暴露在气体中，然后使用可以检测单个原子层分数的微天平来测量表面吸附气体的量。吸附在表面上的原子可以认为是二维物质。它们经历了蒸发和冻结等相变，这与我们熟悉的三维对应物非常相似。当被吸附的原子非常轻且相互作用弱（例如氦）时，量子力学效应变得重要，被吸附的二维液体也可以成为超流体，这意味着它可以在没有任何驱动力的情况下永远流动。理论表明，对于碱金属（锂、钠、钾等）基底上的氦-4，在液-气转变和超-正常流体转变之间存在不寻常的竞争。本次比赛将使用微天平和偏振光反射进行研究。另一种测量表面力的方法是使用原子力显微镜（AFM）。这些力是通过用激光监测微机械硅悬臂梁靠近表面时的挠度来测量的。用浸入液氦的悬臂进行这些测量在技术上是具有挑战性的，这也是这个项目的目标之一。参与该项目的学生和博士后将有机会开发新的仪器，并接受广泛的技术培训，包括低温、高速成像、激光光学和材料制备。这种培训将使他们成为研究界的富有成效的成员，无论是在学术界、工业界还是政府实验室。****技术摘要****该奖项支持在流体，主要是量子流体的一般领域具有几个目标的个人研究者项目。研究了在弱和中等强度碱金属和金属氧化物基体上吸附氦和氦混合物薄膜时的润湿及其与超流体相变的关系，并与理论进行了比较。另一方面，低温原子力显微镜（AFM）技术将得到发展。AFM将进行优化，以便能够测量由于电磁场波动和超流体开始附近阶参量波动而引起的卡西米尔力的距离依赖性。最后，一个较小的项目将研究液滴和气泡中的夹断现象，以阐明常规粘性流体和非牛顿流体中现象之间的联系。为掐断研究开发的计算机代码将适用于研究氦中多电子滴的稳定性和破裂。此外，还将研究超流体液滴在非润湿表面上的运动。参与该项目的学生和博士后将有机会开发新的仪器，并接受广泛的技术培训，包括低温、高速成像、激光光学和材料制备。这种培训将使他们成为研究界的富有成效的成员，无论是在学术界、工业界还是政府实验室。

****NON-TECHNICAL ABSTRACT****Atoms near a solid surface experience a long range attractive force. Long range forces between molecules are important because they determine macroscopic properties of matter; such as what phase (solid, liquid, gas, etc) is stable at a given temperature and the friction between sliding surfaces. Long range forces also determine the shape of large biologically important molecules such as proteins and DNA. This individual investigator award supports project designed to measure long range forces in some simple model systems using several complementary methods. One method is to expose a solid to a gas and then weigh the amount of adsorbed gas on the surface using a microbalance that can detect fractions of a single atomic layer. Atoms adsorbed on a surface can be considered to be two dimensional matter. They undergo phase transitions such as vaporization and freezing, which are very similar to the familiar three dimensional counterparts. When the adsorbed atoms are very light and weakly interacting (e.g. helium), quantum mechanical effects become important and the adsorbed 2D liquid can also become a superfluid, which means that it can flow forever without any driving force. Theory suggests that for helium-4 on substrates of alkali metals (lithium, sodium, potassium, etc) there are unusual competitions between the liquid-vapor transition and the superfluid-normal fluid transition. This competition will be investigated using both microbalances and reflection of polarized light. Another way to measure surface forces is to use an atomic force microscope (AFM). The forces are measured by monitoring the deflection of a micromachined silicon cantilever with a laser when the cantilever is brought near the surface. Making these measurements with a cantilever immersed in liquid helium is technically challenging, and is one of the goals of this project. Students and post doctoral associates involved with this project will have the opportunity to develop novel instruments, and receive training in a broad spectrum of techniques including cryogenics, high speed imaging, laser optics, and materials preparation. This training will enable them to become productive members of the research community, whether in academia, industrial or government laboratories.****TECHNICAL ABSTRACT****This award supports an individual investigator project with several aims in the general area of fluids, predominately quantum fluids. Wetting and its relationship to superfluid phase transitions when films of helium and helium mixtures are adsorbed on weak and intermediate strength alkali metal and metal oxide substrates will be studied and compared with theory. For another study, low temperature atomic force microscope (AFM) techniques will be developed. The AFM will be optimized to enable measurement of the distance dependence of Casimir forces due to both fluctuations in the electromagnetic field and fluctuations in the order parameter near the onset of superfluidity. Finally a smaller project will investigate the pinch-off phenomena in drops and bubbles to elucidate the connections between the phenomena in conventional viscous fluids and non-Newtonian fluids. The computer code developed for the pinch-off studies will be adapted to study the stability and breakup of multi-electron drops in Helium. Furthermore the motion of superfluid droplets on non-wetting surfaces will be studied. Students and post doctoral associates involved with this project will have the opportunity to develop novel instruments, and receive training in a broad spectrum of techniques including cryogenics, high speed imaging, laser optics, and materials preparation. This training will enable them to become productive members of the research community, whether in academia, industrial or government laboratories.